Effectiveness of Seasonal and Pandemic Flu Vaccines

Flu vaccine effectiveness is judged by the ability of the vaccine to generate an immune response (measured by a blood test known as a hemagglutination-inhibition assay; protection is deemed effective if assay titer of antibodies reaches 1:40 or greater). This applies to both seasonal and pandemic flu vaccines. A titer of 1:40 is considered to be protective 21 days after vaccination. Vaccine efficacy also varies from one person to another; one person's titer could be different from another person's titer, even if both were given vaccine from the same batch.

Studies of healthy young adults have shown influenza seasonal vaccine to be
70%-90% effective in preventing illness. In the elderly and those with certain
chronic medical conditions such as HIV, the vaccine is often less effective in
preventing illness. However, studies show the vaccine reduces hospitalization by about
70% and death by about 85% among the elderly who are not in nursing homes. Among
nursing-home residents, vaccine can reduce the risk of hospitalization by about
50%, the risk of pneumonia by about 60%, and the risk of death by 75%-80%.

According to the CDC, early studies with the pandemic H1N1 vaccine showed the injected vaccine to be safe and very effective with immunologic protection developing in healthy people. In the 18-64 age groups, 98% were protected, while in those over 65 years of age, 93% were protected. The fact that H1N1 vaccines were monovalent (only one viral type) and closely matched with most H1N1 viruses may be the reason for the higher effective rates than the trivalent seasonal vaccines. Researchers predicted that two doses (shots) would be needed to get a good immune response, but early studies showed that for most people 10 years of age and older, one dose was effective. In pregnant women, a single dose of the injectable H1N1 vaccine showed protective antibodies in 92% of women after 21 days. However, in children 6 months of age through 9 years of age, two doses of the H1N1 vaccine were recommended by the CDC because early U.S. data suggested that a maximum of 55% of children (36 months of age through 9 years of age) showed protective antibodies 21 days after a single injection, and younger children (6 months of age to 35 months of age) were far less protected (25% protective).

Nasal-spray vaccines can provide effective
protection to individuals 2-49 years of age. The effectiveness is
less in the older population. The adequate administration and the low (0.6%-2.4%) but possible potential for spread to another person makes this vaccine
restricted to healthy individuals 2-49 years of age.

Effectiveness and safety of vaccines are currently being checked by the
cooperative efforts of the FDA, CDC, and other institutions in the U.S. In
December 2009, there was a recall of a batch of pediatric vaccine (800,000 doses
from Sanofi Pasteur) that was manufactured with too little virus antigen.
The remaining doses were recalled. Fortunately, CDC officials said, "No action
(is) necessary by a parent of a child who received this vaccine...(The) only thing
they need to do is make sure their child gets two doses if they're in the 6-month to 35-month age group, which is the group impacted by the recall."
Monitoring of the vaccines is ongoing, and the quadrivalent vaccines will be closely monitored because they are new.